1. Field of the Invention
This invention relates to hydrophilic plastic materials useful as soft contact lenses and, more particularly, relates to hydrophilic copolymers and to their preparation and use, which compositions are extremely resistant to clouding and/or discoloration in use.
2. The Prior Art
As is already known, compositions adapted for use as soft contact lenses have been developed from hydrophilic-type polymers which are softer and more easily accommodated by the eye then are the earlier hydrophobic-type polymers such as polymethyl methacrylate and the like. Hydrophilic polymers may be defined typically as lightly cross-linked, essentially water-insoluble copolymers derived from one or more monomers containing hydroxy groups for imparting to the polymers their affinity for water. These polymers may further be defined as coherent, 3-dimensional polymer structures or networks which have the ability to absorb or imbibe water, even in large quantities, e.g., up to 90 weight %, without dissolution. When containing water in any amount whatsoever, a hydrophilic polymer will expand correspondingly and, in its hydrated state, correctly may be designated as a hydrophilic polymer "gel," or "hydrogel." A specific class of polymer hydrogels which have gained particular commercial acceptance as soft contact lenses are those derived from acrylic esters. U.S. Pat. Nos. 2,976,576 and 3,220,960 issued to O. Wichterle and D. Lim on Mar. 28, 1961, and on Nov. 30, 1965, respectively, are early patents which describe the use of methanol-insoluble hydrophilic acrylic ester polymer materials for the manufacture of soft contact lenses.
Acrylic ester hydrophilic polymers, for the most part, are derived by copolymerizing a mixture containing a major amount of a water-soluble monoester of acrylic or methacrylic acid in which the ester moiety contains at least one hydrophilic group, and a minor amount of a bifunctional diester of acrylic or methacrylic acid which cross-links the hydrophilic group-containing monomer as it polymerizes. The degree and type of cross-linking in the resulting polymer governs, to a large extent, its maximum water content, when fully hydrated.
Although accommodated much more comfortably by the wearer than the prior hard contact lenses, presently known soft contact lenses, as prepared from the aforedescribed polymer hydrogels, do have disadvantageous properties and have not been completely satisfactory. Hydrogel lenses favor the growth of pathological bacteria and fungi on their surfaces. If not regularly cleaned and sterilized or if they are stored in contaminated solutions, pathogens can be easily absorbed by the lens materials due to their flexible, hydrophilic polymer structure. Also, because of their aforesaid flexible, hydrophilic polymer structure, proteins and other normal substances in the eye environment can be easily diffused through the lenses with use. Accumulation of such substances in a soft contact lens causes its discoloration and clouding with repeated cleaning and sterilization techniques practiced by the wearer. Too, the lenses can lose sufficient amounts of water during use to deleteriously affect their dimensional stability and optical acuity.
It is an object of this invention, therefore, to provide a soft contact lens which is resistant to penetration by pathological organisms and chemicals damaging to the eye.
It is another object of this invention to provide a hydrophilic lens polymer which is sufficiently resistant to the diffusion of proteins and other migratory eye substances to prolong its life and optical effectiveness significantly.
It is yet another object of this invention to provide a soft contact lens which will retain a sufficient quantity of water during use to maintain its dimensional stability and optical acuity.